Reference measurements

with ship transits varied from 5,200 to 10,000 cubic

In 1979 the same measurements were made as

yards per ship passage and averaged 7,230 cubic

during 1978 except that a substantial effort was

yards. The maximum was approximately 40,000

applied to measuring the flow distribution around

cubic yards per passage. In the model study the

Sugar Island. The winter measurements were taken

average was 16,100 cubic yards. In terms of the

from 27 February to 3 March 1979, and the data

dimensionless parameter Ar/b2 (area of ice re-

revealed that 67% of the flow went through Little

leased per gap width squared), the average ice

Rapids Cut and 33% of the flow went through

release values are 2.9, 4.3, 2.2 and 3.0 for the four

the North (Lake George) Channel. Summer flow

winters studied (Perham 1985).

measurements were taken from 13 June to 16 June

1979, and the data showed that 71% went down

Little Rapids Cut and 29% went down the North

Reference measurements

The following information is a summary of data

Channel. The flow distribution measurements

obtained by the Detroit District to monitor the

during the winter periods indicate that the vari-

ice boom performance and its effect on levels and

ance of 5% is within the obtainable accuracy range

flows. The primary survey of ice conditions and

for this type of measurement, and therefore no

water levels for the Navigation Season Extension

significant change (effect of the ice boom on river

Demonstration program was conducted by the

flows) is evident (USACE 1980).

U.S. Army Engineer District, Detroit, and consisted

Although the Winter Navigation program

of:

ended after the 1978-79 winter season, it was de-

Field observations using three time-lapse

cided to continue installing the ice boom as part

movie cameras;

of the Soo Area Office, Detroit District regular

Aerial photography of the entire St. Marys

winter operation. The boom system has been of

River but with somewhat greater empha-

value in stabilizing the ice cover in Soo Harbor,

sis on Soo Harbor;

reducing the extent of ice accumulation in Little

Monitoring of water levels in Soo Harbor

Rapids Cut and reducing the amount of ice in

and Little Rapids Cut;

the Sugar Island ferry crossing. These benefits

Discharge measurements taken in the two

occur whether there is winter navigation or not,

channels around Sugar Island to determine

as the harbor ice usually breaks up due to wind

the effect of the ice boom on flow distribu-

and weather several times a year.

tion; and

By lessening the possibility of ice jams in the

Ice thickness measurements throughout the

Cut, the boom has decreased the chances of flood-

winter above and below the boom.

ing in Soo Harbor along with possible power losses

From this information one can tell when the

at the hydropower plants. By reducing the ad-

ice cover began, its area and extent during the

verse effects of natural ice conditions on the Sugar

winter, and some information about the ice edge

Island ferry, it has contributed to more reliable

location in Little Rapids Cut. Some conclusions

transportation between Sugar Island and the

reached during 1978 are:

mainland.

The ice boom helps maintain a fairly stable

ice cover on Soo Harbor, similar to before

Port Huron

winter navigation began, by retaining the

During 1979 a study was initiated to provide

ice that might otherwise move down into

the necessary criteria for the detailed design of

Little Rapids Cut.

an ice control structure at Port Huron (Fig. 21),

Ice in Little Rapids Cut is a major factor in

with the major criteria being structural configu-

retarding flow between Soo Harbor and Fre-

ration and forces. The main problem was that ice

chette Point, but the quantity of ice and

leaves Lake Huron via the St. Clair River, which

degree of retardation do not directly cor-

is typically ice free, and passes through to Lake

relate.

St. Clair, which is usually ice covered. The mov-

The rock-filled scow and crane weights

ing ice accumulates in this stagnant reach just north

proved effective in anchoring the Soo Har-

of Lake St. Clair and often forms deep jams. Two

bor ice field to shore.

physical models were employed: one to evaluate

Ice jamming in and below Little Rapids Cut

hydraulic conditions, the other to evaluate the

is believed to be due to large open-water

effects of wind.

areas in the Cut, frazil ice and heavy snow-

The hydraulic model study used Froude scal-

fall (USACE 1978).

ing with an undistorted geometric scale of 1:85